The objective of this research is an understanding of the nature of the excited electronic states of linear conjugated polyenes and a determination of the role of this excited state structure in the photochemistry of the visual polyenes. In a more general context, this work will provide a firm set of experimental observations which will be used to test various approximate theoretical models of the electronic structure of polyenes and II-electron systems in general. The primary experimental method to be used are high resolution mixed crystal spectroscopy at cryogenic temperatures. Fluorescence, absorption, fluorescence excitation, Raman and two-photon fluorescence excitation spectra will be obtained for a series of dimethylpolyenes, polyene aldehydes and other analogs of the visual pigments. These compounds will be synthesized. Fluorescence lifetime measurements will also play an important role in this study. High power pulsed lasers will be used for two-photon spectroscopy. Emphasis will be placed on a determination of the energy and potential surface of the low lying excited Ag- singlet state of linear polyenes. Normal mode calculations will be used to aid our analysis of vibronic spectra. A high repetition rate picosecond laser system will be used to investigate excited state conformational changes in polyenes.